My hrm watch broke a while back. I continued to used the chest strap with a phone app (Wahoo). Then I got tired of static issues in the winter and super variable data. I used the hrm long enough following reading the article below that I just estimate Z1 and Z2 now by rpe and breath monitoring. I’m sure I’ll get back to using the hrm again to recalibrate my estimation.
https://uphillathlete.com/breath-intensity-monitoring-part-two/